Supramolecular aggregates of photoreactive aromatics. Structure, photophysics and photochemistry of stilbene and azobenzene phospholipids

Abstract

Stilbene and azobenzene-derivatized phospholipids have been synthesized as building blocks to study H-aggregate formation, structure and properties in both bilayer vesicles and LB films. The observed H-aggregates in bilayers display similar spectral characteristics to that in LB films and a strong induced circular dichroism spectrum. Studies of interconversion between H-aggregates and monomers or dimers have established relatively small, integral numbers for aggregate sizes. Based on these results as well as molecular cooling simulation, chiral cyclic pinwheel structures are proposed for these H-aggregates. Single crystal structure of a trans-stilbene derivative ( 4) has been obtained, and its remarkable similarity in structure to compressed monolayer obtained from molecular simulation and almost identical spectra to H-aggregate of stilbene support the proposed herringbone lattice for these aggregates. The very large arrays of aggregates may consist of a “mosaic” of small aggregates. The presence of aggregates as either stable or metastable entities has special consequences for microscopic as well as macroscopic properties of the medium in which they are generated. The aggregates of azobenzene in DPPC vesicles promote reagent release much more efficiently than the correspondent monomer and dimer. Trans—cis photoisomerization of azobenzene aggregates opens a convenient way to establish photoregulatable membrane and related materials.